Ashen had 25 HP when the battle
began, because the Summoning Glyph (which is how you got your weaponry) requires a cost of 75 HP to use. Therefore, he had legitimate reasons to rest.
The others? They didn't want to get afflicted with Tiredness. Nothing is as it seems on Endania
Tiredness actually
is a status effect here that halves damage dealt and doubles damage taken.
... Speaking of which, I probably ought to update the Status Effects list sometime.